Charles Darwin: The Origin of Species by means of Natural Selection

CHAPTER V. LAWS OF VARIATION. 5. COMPENSATION AND ECONOMY OF GROWTH. (continued)

Thus, as I believe, natural selection will tend in the long run to reduce any part of the organisation, as soon as it becomes, through changed habits, superfluous, without by any means causing some other part to be largely developed in a corresponding degree. And conversely, that natural selection may perfectly well succeed in largely developing an organ without requiring as a necessary compensation the reduction of some adjoining part.